Almost definitionally most of the ads would be shown to visitors thought to be bots, which might not impress advertisers, even if G was careful to only count th

Almost definitionally most of the ads would be shown to visitors thought to be bots, which might not impress advertisers, even if G was careful to only count those that passed the CAPTCHAs (though the LLM bots can too, it seems). There is also the reputation risk that advertisers and their brands might become associated with the stress points of having to do a CAPTCHA or being denied access etc. Just two off the top of my head.